Cyclopropylcarbinyl cations substituted

The 2-aryl substituted cyclopropylcarbinyl cations have partial homoallylic character, whose contribution to the resonance hybrid increases when strong electron-withdrawing substituents (e.g. phenyl) are attached at the C2. Thus, 3-arylcyclobutyl tosylates on acetolysis give the homoallylic acetates predominantly, through the intermediate formation of the 2-arylcyclopropylcarbinyl cations (equation 21). 

Several secondary and tertiary substituted cyclopropylcarbinyl cations such as dicyclo-propylmethyl cation, (a-methylcyclopropyl)carbinyl cation and l,a,a-trimethylcyclo-propylcarbinyl cation were discussed in the early review11. 

Olah et al. (1976a) continued the studies of potentially degenerate secondary cyclopropylcarbinyl cations incorporated into polycyclic carbon skeletons with the preparation of 2,4-dehydro-5-homoadamantyl cation [144 R = H]. This ion and two 5-substituted tertiary derivatives were prepared from the corresponding 5-R-2,4-dehydrohomoadamantyl alcohols [145 R = H, CH3... 

Numerous methyl-substituted cyclopropylcarbinyl cations have been observed by NMR as long-lived species, including 6 74 (Table In the case of 68 there was... 

It is of interest to note that artemisia alcohol (18) produced in the hydrolysis of 16-OPy I" is essentially completely racemic (>98%) (57). Apparently (18) is formed by nucleophilic capture of the acyclic allylic carbonium ion (29) rather than direct attack in the 3 position of the chrysanthemyl carbonium ion (28). Nucleophilic substitution upon cyclopropylcarbinyl cations to give homoallyl products occurs with inversion of configuration 74—75). In the case of (28), however, position 3 is highly hindered by the adjacent gem dimethyl groups thus collapse to the allylicly stabilized (29) is faster than direct substitution. The formation of a small amount of cw-chrysanthemol (27, 0.25%) is taken to indicate that allylic ion (29) recyclizes, at least in part, back to (28) and its cis isomer (30). 

The rearrangement of the cyclopropylcarbinyl chloride in solution is well known in the literature (//). In polar solvents three products, arisen from the nucleophilic substitution of the solvent to the chloride, are usually detected, which are formed via nucleophilic substitution of chloride by solvent. This chemistry can be explained by the formation of the bicyclobutonium cation (C4H7+), which acts as a tridentated ion, generating the three products shown in scheme 3. 

Access to homotropenylium ions can be achieved by two general routes. The first involves the addition of an electrophile to a cyclooctatetraene or cyclooctatetraene derivative, an approach which can be considered to correspond to a homoallyl route (Scheme 4). In this route the electrophile is generally attached stereoselectively to the endo position on C(8)18 7,1 74. The second approach involves the ionization of a bicyclo[5.1.0]octadienyl derivative. This is the cyclopropylcarbinyl approach (Scheme 4). This route has the potential of generating a wide range of differently substituted cations however, the starting materials can be difficult to access75 . 

Substituted cyclopropylcarbinyl systems that are free to adopt the optimum conformation around the bond from cyclopropyl to the cationic center are considred in this section. 

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